Page 135 - Chalcogenide Glasses for Infrared Optics

112 Cha pte r F o u r

so because a bar or two were rejected when examined close to the
beginning of the process. All chalcogenide glasses measured at AMI
have had a rupture modulus greater than 2000 psi but less than
3000 psi. The highest has been Amtir 1, as you would expect, at
about 2700 psi. The value was confirmed twice in a Navy study. A
different approach was taken at TI; plates were broken instead of
glass bars. A set of metal concentric rings was used in place of bars
in determining the rupture modulus. Also, the smaller-diameter
upper ring was connected using a swivel joint to ensure force was
applied evenly to the surface of the plate. Figure 4.16 shows photo-
graphs of a two 5.5-in-diameter plates of TI 1173 after being broken.
One was 0.5 in thick and the other 0.25 in. The fracture patterns are
clear in the photographs.

4.3.4 Thermal Conductivity
The absolute measurement of thermal conductivity for a solid is no
easy task. Such a statement applies strongly for materials with low
values. All glasses are considered poor thermal conductors because
they are disordered solids. Chalcogenide glasses are even worse than
oxide glasses. For many years, AMI has used the Thermal Comparator
instrument developed by the Thermophysical Properties Research
Center at Purdue University. A photograph of the instrument is seen
in Fig. 4.14. The controls and microvoltmeter are the main unit while
the thermal probe unit is on the right side. Also shown in the photo-
graph is the acoustical thickness gauge unit to the left. The thermal
probe has a thermal reservoir heated to 10 to 30°C above room tem-
perature. The voltmeter is zeroed. A sample is placed over the hole in
the sensing unit, and the lever is pushed down, which mechanically
brings the probe in contact with the bottom of the sample and the
Constantan tube leading from the heat reservoir. Heat begins to transfer
from the reservoir to the sample. Chromel thermocouples on top of

Picture taken immediately after fracture, With top plate removed,
5.5 in diameter, 0.5 in thick 5.5 in diameter, 0.25 in thick
FIGURE 4.16 Rupture modulus measurement of TI 1173 glass plates.